Heterologous Biosynthesis of Kauralexin A1 in Saccharomyces cerevisiae through Metabolic and Enzyme Engineering

Kauralexin A1 (KA1) is a key intermediate of the kauralexin A series metabolites of maize phytoalexins. However, their application is severely limited by their low abundance in maize. In this study, an efficient biosynthetic pathway was constructed to produce KA1 in Saccharomyces cerevisiae. Also, metabolic and enzyme engineering strategies were applied to construct the high-titer strains, such as chassis modification, screening synthases, the colocalization of enzymes, and multiple genomic integrations. First, the KA1 precursor ent-kaurene was synthesized using the efficient diterpene synthase GfCPS/KS from Fusarium fujikuroi, and optimized to reach 244.36 mg/L in shake flasks, which displayed a 200-fold increase compared to the initial strain. Then, the KA1 was produced under the catalysis of ZmCYP71Z18 from Zea mays and SmCPR1 from Salvia miltiorrhiza, and the titer was further improved by integrating the fusion protein into the genome. Finally, an ent-kaurene titer of 763.23 mg/L and a KA1 titer of 42.22 mg/L were achieved through a single-stage fed-batch fermentation in a 5 L bioreactor. This is the first report of the heterologous biosynthesis of maize diterpene phytoalexins in S. cerevisiae, which lays a foundation for further pathway reconstruction and biosynthesis of the kauralexin A series maize phytoalexins.

Antimicrobial, Cytotoxic and Oxidative Stress Inhibitory Activities of Terpenoids and Flavonols from Senegalia nigrescens (Oliv.) P.J.H. Hurter

Senegalia nigrescens (knob thorn) is a deciduous tree distributed in savannah regions from Tanzania to South Africa used for timber but also medicinally for the treatment of convulsions, wounds, and skin problems. In this study, the biological activities of six phytocompounds, namely: 3 β -hydroxy-20(29)-en-lupan-30-al (1), 30-hydroxylup-20(29)-en-3 β -ol (2), ent-kaur-15-en-18,20-diol (3), melanoxetin (4), quercetin (5) and quercetin-3-O-methyl ether (6), isolated from S. nigrescens were investigated. The compounds were screened against two bacterial (Escherichia coli and Staphylococcus aureus) and one fungal (Candida albicans) strain and were also tested for their cytotoxicity on breast cancer (MDA-MB-231) and normal murine macrophage (RAW 264.7) cell line. Effects of the compounds on attenuating the lipopolysaccharide (LPS)-induced intracellular reactive oxygen species (ROS) production in RAW 264.7 cells were quantified with the H₂DCF-DA assay. This study revealed that flavonols (5 and 6) had the strongest antibacterial and antifungal effects, both having MIC values of 62.5, 31.25 and 31.25 µg/mL on E. coli, S. aureus and C. albicans, respectively. Compounds 2, 3 and 6 were the most cytotoxic against the breast cancer cells with IC₅₀ values of 11.86, 12.62 and 14.03 µg/mL, respectively, while the least toxicity towards normal cells were observed in compounds 2, 5 and 6. All compounds (1-6) significantly lowered ROS production in RAW264.7 cells. In conclusion, tested compounds represent potential promising candidates as antimicrobial, anticancer and antidotes for LPS-induced oxidative stress. This is the first report on the antifungal, cytotoxicity and antioxidative activities of the ent-kaurene diterpenoid, ent-kaur-15-en-18,20-diol (3).

Biosynthesis of the Calorie-Free Sweetener Precursor ent-Kaurenoic Acid from CO2 Using Engineered Cyanobacteria

To supply the sustainable calorie-free sweetener stevioside, synthetic photosynthetic bacteria were developed to produce ent-kaurenoic acid as a precursor of stevioside directly from CO₂. By the use of a combinatorial and modular approach for gene expression, including a cytochrome P450 and the corresponding reductase, engineered Synechoccous elongatus PCC 7942 as a model cyanobacterium enabled the biosynthesis of ent-kaurenoic acid at 2.9 ± 0.01 mg L^-1 from CO₂. We found that the order of genes for expression was critical, producing ent-kaurenoic acid by balancing gene expressions and accumulation of the toxic intermediate in a cell. The engineered bacteria allowed the complete biosynthesis of ent-kaurenoic acid, and it will be used for stevioside biosynthesis from CO₂ as a controlled fermentation.

Engineering Aspergillus nidulans for heterologous ent-kaurene and gamma-terpinene production

Terpenes are a large and varied group of natural products with a wide array of bioactivities and applications. The chemical production of industrially relevant terpenes can be expensive and time-consuming due to the structural complexity of these compounds. Here, we studied Aspergillus nidulans as a heterologous host for monoterpene and diterpene production. Previously, we identified a novel diterpene gene cluster in A. nidulans and showed that overexpression of the cluster-specific transcription factor (pbcR) led to ent-pimara-8(14),15-diene (PD) production. We report further characterization of the A. nidulans PD synthase gene (pbcA). In A. nidulans, overexpression of pbcA resulted in PD production, while deletion of pbcA abolished PD production. Overexpression of Fusarium fujikuroi ent-kaurene synthase (cps/ks) and Citrus unshiu gamma-terpinene synthase resulted in ent-kaurene and gamma-terpinene production, respectively. A. nidulans is a fungal model organism and a close relative to other industrially relevant Aspergillus species. A. nidulans is a known producer of many secondary metabolites, but its ability to produce heterologous monoterpene and diterpene compounds has not been characterized. Here, we show that A. nidulans is capable of heterologous terpene production and thus has potential as a production host for industrially relevant compounds. The genetic engineering principles reported here could also be applied to other Aspergilli.

Hormonal diterpenoids derived from ent-kaurenoic acid are involved in the blue-light avoidance response of Physcomitrella patens

Gibberellins (GAs) are diterpenoid hormones that regulate growth and development in flowering plants. The moss Physcomitrella patens has part of the GA biosynthetic pathway from geranylgeranyl diphosphate to ent-kaurenoic acid via ent-kaurene, but it does not produce GA. Disruption of the ent-kaurene synthase gene in P. patens suppressed caulonemal differentiation. Application of ent-kaurene or ent-kaurenoic acid restored differentiation, suggesting that derivative(s) of ent-kaurenoic acid, but not GAs, are endogenous regulator(s) of caulonemal cell differentiation. The protonemal growth of P. patens shows an avoidance response under unilateral blue light. Physiological studies using gene mutants involved in ent-kaurene biosynthesis confirmed that diterpenoid(s) regulate the blue-light response. Here, we discuss the implications of these findings, and provide data for the ent-kaurene oxidase gene-disrupted mutant.